Implantable cardiac stimulation devices are well known in the art. They include implantable pacemakers which provide stimulation pulses to cause a heart, which would normally or otherwise beat too slowly or at an irregular rate, to beat at a controlled normal rate. They also include defibrillators which detect when the atria and/or the ventricles of the heart are in fibrillation and apply cardioverting or defibrillating electrical energy to the heart to restore the heart to a normal rhythm. Implantable cardiac stimulation devices may also include the combined functionalities of a pacemaker and a defibrillator.
As is well known, implantable cardiac stimulation devices sense cardiac activity for monitoring the cardiac condition of the patient in which the device is implanted. By sensing the cardiac activity of the patient, the device is able to provide cardiac stimulation therapy when it is required.
Unfortunately, noise within the sensing channel bandwidth can interfere with the sensing function. In a pacemaker, for example, noise can be mistaken by the device for a legitimate cardiac event causing stimulation inhibition. This can lead to long periods of asystole. In an implantable defibrillator, noise can cause mistaken diagnosis of fibrillation resulting in inappropriate therapy delivery.
Common sources of noise are either external, such as power line noise, or internal, such as noise from muscle activation. These sources present a particular problem because they generally pass through the sensing bandpass filter with little or no attenuation.
Noise detection systems for use in implantable cardiac stimulation devices are known in the art. However, prior art noise detection systems generally contemplate the use of long refractory periods wherein cardiac events occurring during the refractory period are not acted upon by the device because of the refractory period extension. This can cause the devices in which such prior art noise detection systems are employed to miss normal activations or cardiac events which occur simultaneously with the noise. The present invention addresses this and other deficiencies of prior art noise detection systems.